Dynamic network and routing method for a dynamic network

ABSTRACT

The invention relates to a dynamic network with a plurality of nodes, in which it is provided that—routing information is stored in local routing tables in nodes of the network,—the nodes send an update request to other nodes for updating the local routing tables, and—the addressed nodes send an update response with updated routing information to the requesting nodes.

The invention relates to a dynamic network and to a routing method for adynamic network.

A dynamic network is understood to be a network whose topology canchange dynamically during operation. This includes in particular ad hocnetworks. An ad hoc network is understood to be a self-organizingnetwork in which the structure and the number of participants is notlaid down within given limit values. For example, a communication deviceof a participant may be taken from the network or included in thenetwork. In contrast to traditional mobile telephone networks, an ad hocnetwork is not based on a fixedly installed infrastructure.

Dynamic networks, however, may alternatively be, for example, Internetnetworks whose topology changes during operation.

Such an ad hoc network is known from the book: C. E. Perkins, Ad HocNetworking, Addison Wesley, pp. 53–62. Each node in this known networksends updates of the routing information to adjoining nodes at regularintervals so as to adapt the routing to changes in the network topology.

It is an object of the invention to provide a network of the kindmentioned in the opening paragraph which renders possible an improvedrouting in the case of changes in the network topology. It is a furtherobject of the invention to indicate a relevant routing method.

As regards the network, the object is achieved by means of a dynamicnetwork with a plurality of nodes, in which it is provided that

-   routing information is stored in local routing tables in nodes of    the network,-   the nodes send an update request to other nodes for updating the    local routing tables, and-   the addressed nodes send an update response with updated routing    information to the requesting nodes.

In the network according to the invention, routing information is storedin nodes of the network. Routing information is preferably stored inevery node in the case of decentralized networks. Routing information ispreferably stored in the central nodes only in the case of clusternetworks with central controllers.

The routing information is stored in the form of routing tables. Therouting table of a node preferably comprises fields for all other nodesof the network or for those nodes which are accessible from the node inquestion. The nodes accessible from a given node, i.e. to which atransmission is possible or desired, are denoted target nodes.

The routing information stored in the fields, for example of the nextnode via which a data transmission to the respective target node is totake place (next hop), may be the path length to the target node and themaximum transmission capacity to the target node.

To keep the local routing tables up to date, the nodes having a routingtable preferably send an update request to other nodes at regularintervals. These other nodes are in particular adjoining nodes. They arein particular adjoining controllers in the case of cluster networks withcentral controllers.

The update request signals to the nodes receiving this request that theyshould send updated routing information to the requesting nodes.

The advantage of the requesting mechanism is in particular that therequesting mechanism renders possible a combined transmission of therouting information. The individual nodes transmit modified routinginformation upon request only. In the case of a request, severaltopology changes, i.e. the topology changes that have occurred in thetime period between two requests, may then be sent jointly to therequesting nodes. In a single protocol data unit (PDU), accordingly,several changes in the network topology can be sent to the requestingnodes. This leads to a reduction in the number of PDUs (packets) whichare to be transmitted for the routing purposes.

The invention is based on the idea that the quantity of data to betransmitted for updating the local routing tables can be reduced in thatthe individual nodes request routing information from other nodes.

The advantageous embodiment of the invention as defined in claim 2 isbased on the idea that it is notified to the addressed nodes how up todate the routing information of the requesting nodes is. This renders itpossible for the addressed nodes to make a selection of routinginformation which is to be sent to the requesting node. Only thoserouting data are transmitted to the requesting node each time which aremore recent than the routing information of the requesting node up tothat moment.

For this purpose, the local routing tables contain a table updateinformation and a field update information. The table update informationcontains data on how up to date the local routing table is, i.e. whenthe latest change in the routing table was applied. This may be, forexample, a time indicator or a sequence number. The field updateinformation contains data on how up to date the individual fields of therouting table are, i.e. when the latest change was applied in therespective field of the routing table. The field update information mayagain be, for example, a time indicator or a sequence number. The tableupdate information thus corresponds to the most recent field updateinformation of the respective routing table.

The update request contains the table update information of therequesting node. This allows the addressed node to make a selection ofthe routing information to be sent to the requesting node. The addressednodes know from the table update information of the requesting node howup to date the routing table of the requesting node is, i.e. when themost recent change was made in the routing table of the requesting node.The addressed nodes send an update response containing only that localrouting information which is more recent than the table updateinformation to the requesting node. If the table update information is atime indicator, only those routing data are transmitted which are morerecent in time. The selection of the routing information may take placethrough comparison of the table update information of the requestingnode with the individual field update information of the fields of therouting tables of the addressed nodes. Such a selection of thetransmitted routing information reduces the data quantity to betransmitted for the routing between the individual nodes. Efficientrouting methods can be implemented in this manner.

The transmission of the table update information to the addressed nodeshas the advantage that the table update information requires only littletransmission capacity and that only one item of table update informationis to be transmitted for each routing table or node. The update requestthus occupies only little transmission capacity. This is advantageous inparticular in the case of wireless networks.

In the advantageous embodiment of the invention as defined in claim 3,the local routing tables contain a topology change information. Theup-to-dateness of a change in the network topology can be characterizedby the topology change information, so that it can be indicated when achange in the network topology has occurred in the network. Preferably,each field in the local routing tables contains an item of topologychange information. This may be, for example, a time indicator or asequence number. Since it takes some time until the information relatingto a change in the network topology has been distributed over theindividual nodes of the network, it can be distinguished in each nodereceiving this information later in time whether this information is newto it, whether it has stored this information already, or whether thisinformation is already obsolete for it, for example because it hasalready received a more recent item of information from a differentnode. The topology change information thus renders it possible to carryout efficient updates of the local routing tables.

This is achieved in the advantageous embodiment of the invention asdefined in claim 4 in that the update response comprises the topologychange information, and that an update of the individual fields of thelocal routing tables is carried out if the topology change informationof the addressed node is more recent than the topology changeinformation of the requesting node. The node which has sent an updaterequest to adjoining nodes may thus carry out a selective update afterreceiving the replies. An update of the individual fields of therequesting node is carried out if a more up to date or more recentinformation on the network topology is obtained thereby.

If the topology change information of the addressed node is as recent asthe topology change information of the requesting node, according to theadvantageous embodiment of the invention as defined in claim 7, anupdate will be carried out if the path length to the respective targetnode is made shorter by the update.

In the advantageous embodiment of the invention as defined in claim 8,an update is carried out if the topology change information of theaddressed node is as recent as the topology change information of therequesting node, and the maximum data transmission rate to therespective target node is made-greater by the update.

The advantageous embodiments of the invention as defined in claims 7 and8 may also be combined, in which case the criterion of claim 7 or thecriterion of claim 8 may be given a higher priority, depending on theapplication.

The embodiment of claim 5 has the advantage that time informationrenders possible an independent comparison of the up-to-dateness of theindividual items of routing information.

The embodiment having sequence numbers as defined in claim 6 can berealized in a particularly simple manner.

It is advantageous in the case of central cluster networks which have acentral node (controller) for controlling the clusters (sub-networks) tostore the routing table for the respective sub-network centrally in thecentral node. The transmission of information between the individualsub-networks takes place via bridge nodes or forwarders.

The object of the invention relating to the method is achieved by meansof a method having the characteristics of claim 10.

A few embodiments of the invention will now be explained in more detailbelow with reference to the drawing comprising FIGS. 1 to 6, in which:

FIG. 1 shows a routing table with routing information,

FIG. 2 shows a dynamic network with 5 sub-networks, each controlled by acentral controller, at a first moment in time,

FIG. 3 shows the network of FIG. 2 at a second moment in time, with anode of the network shifted to an adjoining sub-network as compared withFIG. 2,

FIG. 4 is a table showing the time sequence of the change of the routingtables of the central controller as a result of the shift of the networknode,

FIG. 5 shows part of a time register, and

FIG. 6 shows a network with 5 sub-networks which are each controlled bya central controller.

FIG. 1 shows a routing table which is stored in the routing node orstation of a network. The routing table comprises fields T1 to TN foreach station of the overall network as well as a time t_(up) as a tableupdate information, i.e. the time at which the routing table was changedfor the last time. The fields T1 to TN of each individual station of thenetwork comprise 6 sub-fields 1 to 6. The first sub-field 1 contains theidentification number (ID) of the respective target station. The secondsub-field 2 stores the ID of that station to which the data destined forthe target station of the first sub-field 1 are to be passed on. It isthus always stored for each possible target station which is theso-called “next hop” on the way to this target station. If the targetstation itself is the next hop, the target station itself is entered inthe second sub-field 2. The third sub-field 3 stores the generation timet_(gen) of the field as a topology change information. The generationtime t_(gen) indicates when a change has occurred in the networktopology for this field, i.e. for this target station. It is laid downby that station which detects the change in the network topology andsubsequently implements a change in the contents of the field in itslocal routing table. The fourth sub-field 4 contains a time t_(reg).This indicates when a change in the contents of a field was adopted bythe respective station in the routing table owing to a change in thenetwork topology, or when the changes notified by adjoining stationswere adopted. The fifth sub-field 5 of each field contains a quantitysuch as, for example, the remaining path length up to the targetstation. Further quantities may be stored in additional sub-fields ofeach field. The sixth sub-field contains the maximum data rate withwhich data can be transmitted to the target station. This data ratecorresponds to the minimum data rate of all constituent paths from therelevant station to the target station.

An UPDATE procedure is provided for refreshing the routing tables so asto keep the routing tables up to date at all times. This UPDATEprocedure is based on a request-response mechanism. Each routing stationperiodically initiates this UPDATE procedure. The station, referred tobelow as the UPDATING station (US), transmits an UPDATE REQUEST messagein the broadcast mode to its immediate (routing) neighbor stations. TheUPDATE REQUEST message contains the time t_(up), i.e. the moment of themost recent table update of the requesting station.

Upon reception of the UPDATE REQUEST, the neighboring stations comparethe received time t_(up) with the registration times t_(reg) of eachindividual entry in their own routing tables. When this comparison hasbeen completed, each neighboring station sends an UPDATE RESPONSEmessage (possibly segmented) to the requesting station US, in whichmessage all those entries of the respective routing table are includedwhich were entered into the table after the moment t_(up), i.e. forwhich it is true that t_(reg)>t_(up). It should be noted that the timet_(up) should generally be converted to the clock system of therespective neighboring station before the times t_(reg) and t_(up) canbe compared. This circumstance and the steps made necessary thereby willbe clarified after the description of the general routing procedure. Allfields are transmitted for each of the entries to be transmitted, exceptfor the next hop terminal ID and the registration time t_(reg), becausethese are irrelevant for the requesting station US.

Upon reception of the UPDATE RESPONSE, the US compares the generationtimes and quantities of the entries received with the generation timesand quantities of the entries present at that moment in its own routingtable.

The newly received entries or fields are now denoted “new”, and theentries of the requesting station obtaining until that moment aredenoted “US”. “PL” denotes the path length, as is apparent in FIG. 1,and “MTR” the maximum transmission rate. MTR^(US-NS) denotes the maximumtransmission rate between the requesting and the replying neighboringstation. By analogy, PL^(US-NS) denotes the number of hops between therequesting station US and the neighboring station NS. This is because itis conceivable that not all stations provide routing data. It could thushappen that two routing stations, which are neighbors in the sense ofthe routing procedure, communicate with one another via one or severalnon-routing stations.

According to the invention, the US only carries out those entries whichfulfill the following criteria:

t_(gen)^(new) > t_(gen)^(US)or  (t_(gen)^(new) = t_(gen)^(US)  and  P L^(new) + PL^(US − NS) < PL^(US))or  (t_(gen)^(new) = t_(gen)^(US)  and  P L^(new) + PL^(US − NS) = PL^(US)  and  min (MTR^(new), MTR^(US − NS)) > MTR^(US))

Once the UPDATE criteria for a received entry have been fulfilled, a fewfields of the contents until that moment are replaced as follows:

-   the next hop terminal ID is replaced with the ID of the relevant    neighboring station.-   the generation time of the new entry is adopted:

t_(gen)^(US) = t_(gen)^(new)

-   the new path length is: PL^(US)=PL^(new)+PL^(US-NS)-   the new maximum data rate is: MTR^(US)=min(MTR^(new), MTR^(US-NS))

In this case, too, the station must convert the time

t_(gen)^(new)into its own time system before time comparisons and replacements can becarried out (see the explanation further below).

FIG. 2 shows a network with 5 sub-networks 10 to 14 at a first momentt0. The sub-networks 10 to 14 are each controlled by a respectivecentral controller CC1 to CC5. The individual sub-networks 10 to 14 caneach be connected via bridge nodes or forwarders F1 to F5. The bridgenode F1 connects the sub-networks 10 and 11, the bridge node F2 thesub-networks 11 and 12, the bridge node F3 the sub-networks 11 and 13,the bridge node F4 the sub-networks 12 and 14, and the bridge node F5the sub-networks 13 and 14. By way of example, a station ST1 is presentin the sub-network 11. The sub-networks 10 to 14 may comprise furtherstations or nodes (not shown) in any manner whatsoever. In addition,FIG. 2 shows the transmission rates of the links between the individualsub-networks as well as between the CC2 and the station ST1. Thetransmission rate between the sub-network 10 and the sub-network 11 viathe forwarder F1 is 10 Mbit/s, the transmission rate between thesub-network 11 and the sub-network 12 via the forwarder F2 is 5 Mbit/s,the transmission rate between the sub-network 11 and the sub-network 13via the forwarder F3 is 0.1 Mbit/s, the transmission rate between thesub-network 12 and the sub-network 14 via the forwarder F4 is 1 Mbit/s,and the transmission rate between the sub-network 13 and the sub-network14 via the forwarder F5 is 3 Mbit/s. Finally, the transmission ratebetween the controller CC2 and the station ST1 is 5 Mbit/s. The links inthe network shown by way of example in FIG. 2 thus always pass throughthe central controllers CC1 to CC5.

FIG. 3 shows the network with 5 sub-networks of FIG. 1 at a secondmoment t1. The topology of the network has changed at this secondmoment, i.e. the station ST1 has moved from the sub-network 11 to theneighboring sub-network 10. The transmission rate between the controllerCC1 of the sub-network 10 and the station ST1 now is 10 Mbits/s.

FIG. 4 illustrates the changes in the routing tables of the controllersCC1 to CC5 over time as a result of the shift of the station ST1 fromthe sub-network 11 of FIG. 2 to the sub-network 10 as shown in FIG. 3.

The Table of FIG. 4 contains a column for each of the controllers CC1 toCC5. The fields of the routing tables of the individual controllers forthe target station ST are indicated in the columns at five differentmoments t0 to t4 for the controllers CC1 to CC5. The fields in thisexample each have 5 sub-fields. The sub-field provided in accordancewith FIG. 1 containing the identification number (ID) of the respectivetarget station is not shown in FIG. 4, because FIG. 4 shows routinginformation relating to the target station ST1 only.

The upper sub-field of the fields in FIG. 4 indicates the bridge node orforwarder to which the data destined for the station ST1 are to bepassed on. This means that only the so-called next hop on the way tothis target station ST1 is stored each time for this target station ST1.The second sub-field 5 from the top contains the remaining path lengthup to the target station. The central sub-field contains the maximumdata rate with which data can be transmitted to the target station ST1.This data rate corresponds to the minimum data rate of all constituentpaths from the relevant controller up to the target station ST1. Thesecond sub-field from the bottom provides the generation time t_(gen) ofthe field for the target station ST1 by way of topology changeinformation. The generation time t_(gen) indicates when a change hasoccurred in the network topology for the target station ST1. It isregistered by that station which detects the change in the networktopology and subsequently carries out a change in the contents of thefield in the local routing table. In the present case this is thecontroller CC1 of the sub-network 10. The bottom sub-field contains thetime t_(reg). This indicates when the change in the network topology wasincluded into the respective routing table by the respective station,i.e. in this example by the respective controllers, or when the changestransmitted by neighboring stations were adopted.

In the present example, the column contains the fields of the routingtables of the controllers CC1 to CC5 for the target station ST1 and thenetwork in accordance with FIG. 2 at the moment t0.

At the moment t1, the target station ST1 is shifted from the sub-network11 to the sub-network 10. This corresponds to the network topology ofFIG. 3. This is recognized by the controller CC1 of the sub-network 10,and this controller CC1 accordingly changes its routing table at themoment t1. The topology change information has accordingly arisen at themoment t1, which means that t_(gen)=1 is set. The change that hasoccurred in the network topology was also entered by the controller CC1in its routing table at the moment t1 and registered accordingly. Thismeans that t_(reg) is also set for 1. The controllers CC2 to CC5 do notknow the change in the network topology at the moment t1 yet. Thisinformation must first be distributed over the network. This is done bymeans of the requests sent by the individual controllers to theneighboring controllers at regular intervals, and by means of therelevant responses of the controllers thus addressed.

The controller CC2 receives a reply to its request from the controllerCC1 at the moment t2, and the change in the network topology is enteredinto the routing table of CC2. Since the topology change has occurred atthe moment t1, t_(gen)=1 is set. The change in the network topology wasentered by the controller CC2 into its routing table at the moment t2and registered accordingly. This means that t_(reg) is set for 2.

The controllers CC3 and CC4 receive a reply to their requests from thecontroller CC2 at the moment t3, and the change in the network topologyis entered into the routing tables of CC3 and CC4. Since the topologychange has occurred at the moment t1, t_(gen)=1 is set. The change inthe network topology was entered into the routing tables by thecontrollers CC3 and CC4 at the moment t3 and registered accordingly.This means that t_(reg) is set for 3.

The controller CC5 receives a reply to its request from the controllerCC3 and/or the controller CC4 at moment t4, and the change in thenetwork topology is entered into the routing table of CC5. Since thetopology change has occurred at the moment t1, t_(gen)=1 is set. Thechange in network topology was entered into the routing table of therespective controller CC5 at the moment t4 and registered accordingly.This means that t_(reg) is set for 4.

The further sub-fields are also adapted to the changed network topologyat the respective moments t1 to t4 in a corresponding manner.

Additional functions may be implemented in the routing, if so desired.The constant period of refreshing of the routing tables means thattopology changes occurring between two UPDATE moments are notcommunicated immediately to the neighboring stations, but at the nextUPDATE moment. Advantageously, however, particularly important changes,such as the failure of connection paths, may also be communicated to theneighbors without any preceding request. This is done by means of anUPDATE TRIGGER message which contains the relevant entries relating tothe changed fields.

To avoid data on current links being lost in the time between thetopology change and the next UPDATE moment, moreover, the node detectingthe topology change may send an ERROR message to the sources or endterminals of the relevant links in order to stop the current connection.

The moments in time t_(up), t_(gen), and t_(reg) defined in the protocolmay be coded in various ways. An obvious coding relates to an overallsystem clock which is coded as a multiple of a basic clock modulo amaximum value in the form of a bit sequence. The use of an overallsystem clock, however, would require a synchronization of all stationsof the network. Some communication standards (for example the standard1394.1) already achieve a synchronization of all appliances of anetwork, but the availability of an overall system clock cannot be takenfor granted in general. For this reason, the use of an overall systemclock is avoided. The algorithm is in fact already fully functional ifneighboring stations are informed about the difference between theirlocal times or clocks. It is accordingly provided that neighboringstations inform one another of their prevailing local system clocks. Theperiod of this information exchange may usually be chosen to beextremely wide, as will be explained further below. The clockinformation exchange thus represents a negligibly small occupation oftransmission resources. Each station stores the difference between itslocal time and the local time of each individual neighboring station.When a station receives an UPDATE request with the parameter t_(up), itwill add the clock difference stored in relation to the relevantneighboring station to t_(up) so as to convert the moment of the mostrecent change in the routing table of the neighboring station into itsown time system. Then the converted time t_(up) may be compared with theregistration times t_(reg) of the own routing inputs in accordance withthe normal routing procedure, and an UPDATE response can be generated.When a station receives an UPDATE response to a preceding UPDATErequest, the generation time t_(gen) of each entry received is firstconverted into the local time system exactly as in the preceding case inthat the clock difference with the relevant neighboring station is addedto the time t_(gen). Then it is decided in accordance with the normalrun of the routing algorithm whether the received entry is to beincluded into the own routing table or not.

It should be noted that the clock difference stored by two neighboringstations has opposed signs, i.e. in the case of an exchange of an UPDATErequest and UPDATE response between two neighboring stations theaddition of a positive clock difference value in one station willcorrespond to the subtraction of the same positive value (i.e. theaddition of a negative value) in the other station.

Clock generators usual at present generally have an accuracy in themicrosecond or nanosecond range. Such a high accuracy of the timedefinition is not necessary for the routing procedure under discussionhere. To minimize the number of bits to be transmitted, accordingly, nomore than a fraction of the internal clocks of the stations is used forthe time indicators t_(up), t_(gen), and t_(reg).

FIG. 5 shows by way of example an excerpt from a complete time register.The intervals of the register shown in FIG. 5 correspond to single bits.The time value is dually coded, the significance of the bits increasingfrom right to left, so that the Most Significant Bit (MSB) lies at theextreme left.

The excerpt of the register chosen for the routing procedure, shownhatched in FIG. 5, is determined by the two times T_(max) and T_(min).

The upper limit of the chosen register excerpt determines the maximumtime after which an entry into a routing table must be erased. This isbased on the modulo definition of the time excerpt within the routingprocedure. A station must test each entry already present in the routingtable at each time step (T_(precision)) as to whether the generationtime t_(gen) of the entry corresponds to the current local time (or theexcerpt from the time register). If this is the case, the entry iserased. This is because an old entry would otherwise seem to be highlyup to date again after one modulo period because of the modulodefinition.

The order of magnitude of T_(min) determines the accuracy of the timecoding and bases itself on the minimum period of transmission of theUPDATE request messages. The reason for this is that all entries changedin the responding station since the latest UPDATE of the requestingstation are sent to the requesting station, independently of whether thechanges were implemented shortly or long after the latest UPDATE. Thesame holds for the replacement of entries in the requesting station. Amore accurate coding of the time will give no advantage in this respectand will merely occupy transmission capacity.

Advantageously, however, an excerpt enlarged by a few bits in downwarddirection (up to the time T_(precision) in FIG. 5) is chosen fortransmitting and storing the times t_(up), t_(gen), and t_(reg),although only the bits fully hatched in FIG. 5 are actually processed inthe routing algorithm. Propagation effects of rounding errors in theconversion of the clocks of one timing system into another timing systemcan be avoided in this manner.

The register excerpt is laid down once and for all and cannot be changedduring the operation of individual stations.

The same is not true, however, for the period of transmission of theUPDATE request messages. The routing method does not require allstations to use the same period. This is utilized in the sense that eachstation optimizes its own UPDATE period during operation. Empty UPDATEresponse messages may, for example, point to the fact that the UPDATEperiod can be expanded. High topology change rates and subsequentconnection interruptions and packet losses should lead to a reduction ofthe UPDATE period. The routing method thus automatically adapts itselfto various system scenarios and mobility rates.

In accordance with the coding instruction shown, the statement madeinitially may now be motivated, i.e. the statement that the informationexchange for determining the clock differences between neighboringstations may take place comparatively seldom: the frequency of theinformation exchange follows the so-termed clock drift of the localclock generator of each station. Usually the clock drift is of the orderof or even a few orders of magnitude lower than the minimum coding levelof the clock register (least significant bit or LSB in FIG. 5). Thelower limit of the excerpt considered, i.e. the time T_(min), however,is chosen to be higher by a few powers of two as shown in FIG. 5. Anexchange of information on the clock differences should then take placeat the latest when a shift of the order of T_(precision) could beachieved on account of the clock drift.

Subsequently, the exchange of information about the clock differences isrepresented for a self-organizing network organized in the form ofsub-networks or clusters. An example of such a network is shown in FIG.6.

In the cluster-based network of FIG. 6, a single station, the centralcontroller (CC), carries out the routing algorithm for all stations ofits own cluster. The network of FIG. 6 comprises five clusters 20 to 24.The clusters 20 to 24 have CCs 30 to 34. This means that only the CCs 30to 34 are neighbors in the sense of the routing method described above.The CCs, however, cannot communicate directly with one another ingeneral, but they must exchange information via so-termed forwardingterminals (FT) which lie in the overlapping regions of the clusters. Theclusters 20 and 22 are connected by means of an FT 40, the clusters 21and 22 by means of an FT 41, the clusters 22 and 23 by means of an FT42, and the clusters 24 and 22 by means of an FT 43. The cluster 20 has,for example, a further station 50, the cluster 21 further stations 51and 52, the cluster 23 further stations 53 and 54, and the cluster 24further stations 55 to 57.

The time or clock information is exchanged between the CCs 20 to 24 inthe following manner: each FT 40 to 43 and each CC 20 to 24 stores acopy of its complete clock register at the start of each MAC frame.Furthermore, each CC periodically (in accordance with the period of theclock information exchange) transmits a broadcast message within itscluster in which the copy of the clock register at the start moment ofthe present MAC frame is contained as a parameter. The FTs receivingthis message form the difference from the copy of their own clockregister and the received copy of the clock register of the CC. In thismanner they determine the shift between the clocks of the CC and theirown clocks and store this difference. Once an FT has determined thedifference with a further CC by the same procedure, it is capable ofdetermining the clock difference of the two CCs by subtracting the twoshift values. The clock difference is subsequently transmitted to thetwo CCs in a signaling message specially designed for this purpose.

1. A dynamic network, comprising: a plurality of nodes for transmittingand receiving information, each of said nodes including local routingtables in which routing information is stored, each of said localrouting tables including a first ield containing a time-based networktopology change information and a second field containing a time-basedindication of when said local routing table has changed as a result ofthe change in the network topology, each of said nodes is arranged tosend an update request to other, addressed ones of said nodes forupdating the local routing tables in said node, the update requestincluding time-based table update information indicative of the mostrecent update of said local routing tables in said requesting node, andeach of said addressed nodes is arranged to send at least one updateresponse with updating routing information to the requesting one of saidnodes only when said local routing tables of said addressed node reflectmore updated routing information than said local routing tables of saidrequesting node as determined by analysis of said second field in saidlocal routing tables of said addressed node and the table updateinformation in the update request, said requesting node being arrangedto consider a change in said local routing table only when the at leastone update response with updating routing information being sent by oneof said addressed nodes indicates a later change in the network topologyor an equally up-to-date network topology and a more desirable datatransmission.
 2. A network as claimed in claim 1, wherein said localrouting tables contain the time-based table update information relatingto the most recent updating of said local routing table and said secondfield in said local routing tables reflects field update informationrelating to the most recent updating of the individual fields, theupdate response containing those items of local routing information forwhich the field update information of said addressed node is more up todate than the table update information of said requesting node.
 3. Anetwork as claimed in claim 1, wherein said first field characterizesthe up-to-dateness of a change in the network topology.
 4. A network asclaimed in claim 3, wherein said nodes are arranged such that the updateresponse generated by addressed one of said nodes contains said firstfield of said local routing tables of said addressed node andcharacterizes the topology change information, said requesting nodebeing arranged to consider a change in said local routing table onlywhen the at least one update response with updating routing informationbeing sent by one of said addressed nodes indicates a later change inthe network topology as determinable by analysis of said first field insaid local routing table in said requesting node and said first fieldcontained in the update response with updating routing information.
 5. Anetwork as claimed in claim 1, wherein the contents of said first andsecond fields are items of time information.
 6. A network as claimed inclaim 1, wherein the contents of said first and second fields areconstituted by sequence numbers.
 7. A network as claimed in claim 1,wherein said requesting node after receiving the update responses fromone of said addressed nodes is arranged to carry out an update of itslocal routing table only when the at least one update response withupdating routing information being sent by one of said addressed nodesindicates a network topology equally up to date as the topology changeinformation of said requesting node and a path length to a respectivetarget node is made shorter by the update.
 8. A network as claimed inclaim 1, wherein said requesting node after receiving the updateresponses from one of said addressed nodes is arranged to carry out anupdate of its local routing table only when the at least one updateresponse with updating routing information being sent by one of saidaddressed nodes indicates a network topology equally up to date as thetopology change information of said requesting node and a maximum datatransmission rate to an envisaged target node is made higher by theupdate.
 9. A network as claimed in claim 1, wherein the network issubdividable into several sub-networks which each contain a controllerfor controlling said sub-networks, said sub-networks beinginterconnectable by means of respective forwarding terminals, and eachcontroller being designed for storing and managing a central routingtable for a respective one of the sub-networks.
 10. A network as claimedin claim 1, wherein each of said local routing tables include a thirdfield containing table update information relating to the most recentupdate of said local routing table, a fourth field containing anidentification of a target node, a fifth field containing anidentification of an adjoining node in a path toward the target node,and a sixth field containing a quantity representing a path length tothe target node.
 11. A network as claimed in claim 10, wherein each ofsaid local routing tables further includes a seventh field containing amaximum data rate at which data is transmittable to the target node. 12.A network as claimed in claim 1, wherein the time-based table updateinformation and the contents of said first and second fields are coded.13. A network as claimed in claim 1, wherein each of said nodes whenacting as an addressed node is arranged to convert the time-based tableupdate information contained in the update request into its own timesystem before comparing the time-based table update information to saidsecond field in said local routing tables of said addressed node.
 14. Anetwork as claimed in claim 1, wherein each of said nodes when acting asa requesting node is arranged to convert the time-based information insaid first field contained in the at least one update response withupdating routing information into its own time system before comparingsaid second field of said local routing tables of said addressed nodewith said second field of said local routing tables of said requestingnode.
 15. A network as claimed in claim 1, wherein each of said nodes isarranged to generate and send the at least one update response bycombining information about a plurality of network topology changes in asingle update response.
 16. A routing method for a dynamic networkhaving a plurality of nodes, comprising: storing routing information inlocal routing tables in each node of the network, each of the localrouting tables including a first field containing a time-based networktopology change information and a second field containing a time-basedindication of when the local routing table has changed as a result ofthe change in the network topology, sending an update request from eachof the nodes to other, addressed one of the nodes for updating the localrouting tables in that node, the update request including time-basedtable update information indicative of the most recent update of thelocal routing tables in the requesting node, sending at least one updateresponse with updating routing information from each of the addressednodes only when the local routing tables of the addressed node reflectmore updated routing information than the local routing tables of therequesting node as determined by analysis of the second field in thelocal routing tables of the addressed node and the table updateinformation in the update request, and changing the local routing tablesin the requesting one of the nodes only when the at least one updateresponse with updating routing information being sent by one of theaddressed nodes indicates a later change in the network topology or anequally up-to-date network topology and a more desirable datatransmission.
 17. A method as claimed in claim 16, wherein the localrouting tables in the requesting one of the nodes are changed only whenthe at least one update response with updating routing information beingsent by one of the addressed nodes indicates a later change in thenetwork topology.
 18. A method as claimed in claim 16, wherein the localrouting tables in the requesting one of the nodes are changed only whenthe at least one update response with updating routing information beingsent by one of the addressed nodes indicates an equally up-to-datenetwork topology and a path length to a respective target node is madeshorter by the update or a maximum data transmission rate to anenvisaged target node is made higher by the update.
 19. A node for adynamic network, comprising: local routing tables in which routinginformation is stored, each of said local routing tables including afirst field containing a time-based network topology change informationand a second field containing a time-based indication of when the localrouting table has changed as a result of the change in the networktopology, the node being arranged to generate and send an update requestto other nodes which respond to the update request by sending at leastone update response with updated routing information, the update requestincluding time-based table update information indicative of the mostrecent update of said local routing tables in the requesting node, thenode being arranged to consider a change in said local routing tableonly when the updating routing information of the at least one updateresponse sent by the other nodes indicates a later change in the networktopology or an equally up-to-date network topology and a more desirabledata transmission.
 20. A node as claimed in claim 19, wherein the nodeis arranged to receive update requests from other nodes and send atleast one update response with updating routing information in responseto the update request to the requesting node only when the contents ofsaid second field of said local routing tables of said node indicatesthat said local routing tables of said node reflect more updated routinginformation than said local routing tables of the requesting node asdetermined by analysis of said second field in said local routing tablesof the node and said second field of said local routing tables of therequesting node as sent in the update request.